For cart orders that include any tangible items (products with Shipping & Buyer's Address enabled) the PayPal checkout button in our shopping cart still brings up the standard PayPal checkout because PayPal has not yet made their mobile-optimized checkout compatible with their Tax/Shipping Recalculation feature, which is necessary to make sure tax/shipping charges are accurate for the actual shipping address buyers provide during checkout.
When we contacted with them to inquire when we could expect them to start supporting that feature in their mobile checkout, they responded that they have no plans to do so on their current roadmap. That being the case, we are investigating the feasibility of not sending any shipping recalculation parameters to PayPal checkout whenever we detect the buyer is on a mobile device.
Meanwhile, cart orders which do not include any tangible items, or Buy Now buttons for any item (including tangible items), should bring up PayPal's mobile checkout, since in those cases no shipping destination or fee information is being determined in the cart and sent to PayPal checkout.
Also, PayPal Payments Pro uses a different version of PayPal checkout called Express Checkout, which does support shipping recalculation and thus can offer a mobile checkout experience even for orders with tangible items; this help page explains how to set up PayPal Payments Pro with E-junkie: